This invention relates to electromechanical transducers, that is, devices which convert an electrical input into a mechanical output. More particularly, this invention relates to motors and, especially, stepping motors, which convert discrete electrical signals into mechanical torques that act on a mass, such as a shaft, or other mechanical load, to rotate the mass in a series of steplike angular displacements of essentially uniform magnitude. Specifically, this invention is directed to a hybrid stepping motor unit.
Conventional stepping motors have many uses. A stepping motor can be used, for example, to convert a series of digital pulses from a computer into steplike angular displacements of a shaft which is an element of a drive train for positioning a machine tool component. A stepping motor can also be employed to position an optical code disc in an electronic typesetting system or to position the pen of a chart recorder. Other exemplary uses are well known to those who are skilled in the art, several being described in "Sigma Stepping Motor Handbook", published in 1972 by Sigma Instruments, Inc.
Prior art stepping motors are of two basic types. One type of system is the conventional open loop stepper motor. This type of stepping motor is described, for example, in the aforementioned Sigma Instruments, Inc. publication. A second type of system is the conventional closed loop step-servo motor. An example of this type of stepping motor is the Digital Control C200 Series ID0376-SP-3M which is manufactured by Inland Motor Division of Kollmorgen Corporation.
An open loop stepper motor characteristically has a high heat loss to power ratio. The open loop stepper motor is, therefore, impractical for high horsepower applications.
Despite limitation to applications which require only fractional horsepower, however, open loop stepper motors advantageously provide a relatively stiff drive mechanism for a load. Stated differently, in low horsepower applications where open loop stepper motors are generally used, each discrete electrical command signal, under nominal load, is translated into a corresponding steplike displacement of the load. As the load on the open loop stepper motor increases, however, the displacement of the load lags the command signal by an increasing fraction of a step. At full load, the step lag is exactly onehalf step. Any load beyond full load will cause the open loop stepper motor to loose one, or more, steps. Nevertheless, the stiffness of the open loop stepper motor, measured in ounce-inches of torque per step lag, is relatively high compared to that of a closed loop step-servo motor.
The closed loop step-servo motor, on the other hand, characteristically has a lower heat loss to power ratio than the open loop stepper motor. The closed loop step-servo motor is, therefore, generally used in applications which require high horsepower.
In comparison with the open loop stepper motor, however, conventional closed loop step-servo motors provide a more compliant, or less stiff, drive mechanism for a load, since the closed loop step-servo motor itself receives no electrical drive signal until the load displacement lags the electrical command signal by a minimum of one full step. Unlike the open loop stepper motor, which carries full load with a lag of only one-half step, the closed loop step-servo motor carries only the first increment of load with a lag of one full step. Moreover, in order to provide a smooth progression of torque increments, the closed loop step-servo motor requires serveral full step lags in order to develop full torque. Consequently, the drive stiffness of the closed loop step-servo motor, measured in ounce-inches of torque per step lag, is low relative to that of the open loop stepper motor. This low drive stiffness of the closed loop step-servo motor has a highly detrimental effect on the resolutional integrity under load compared to the resolutional integrity under load of an open loop stepper motor.
In summary, open loop stepper motors have high heat loss to power ratios and are, therefore, suitable only for fractional horsepower applications. Open loop stepper motors do, however, have relatively high drive stiffness which enables them to maintain high resolutional integrity under load, which also puts a minimal demand on the source of electrical command signals, such as a computer. Closed loop step-servo motors, on the other hand, have relatively low heat loss to power ratios and are, therefore, suitable for multihorsepower applications. However, closed loop step-servo motors have low drive stiffness which does not enable them to maintain good resolutional integrity under load, which puts a high demand on the computer or other electrical command signal source.